There is much information, both clinical and experimental, to suggest that decreased thyroid function adversely affects the development and course of breast cancer. Most of the published work has been based on superficial or indirect assessment of thyroid function, leading to considerable question about the real significance of this relationship. Despite this shaky foundation, thyroid hormone is used widely as part of the therapy in human breast cancer. In this study, we propose to use newer techniques of hormone assay and methodology to study: (1) the control in experimental animals of TSH and prolactin secretion and how they are related to thyroid function; (2) the effects of changes in thyroid function on the development and course of mammary carcinoma in rats; and (3) the incidence of abnormal thyroid function in women with breast cancer. BIBLIOGRAPHIC REFERENCE: Augustine, E. C. and Macleod, R. M. Prolactin and Growth Hormone Synthesis: Effects of Perphenazine, alpha-Methyltyrosine and Estrogen in Different Thyroid States. Proc. Exp. Biol. Med. 150, 551-556, 1975.